Strippable layer relief printing

ABSTRACT

Overcoated layer relief electrostatic printing in which a latent electrostatic image is made visible by the deformation of a compliant layer. The relief deformation occurs in a thermoplastic layer superimposed on conventional xerographic materials such as the conductive substrate which has been coated with a photoconductive insulating layer, the thermoplastic material overcoating the photoconductive layer. Apparatus is disclosed for separable and permanent thermoplastic overcoating on the xerographic plate. An interlayer between the thermoplastic overcoating and the photoconductive layer is included which serves as a deformable support and to protect the photoconductive layer from any interaction between the particular thermoplastic used and the solvent or heat used to initiate the thermoplastic deforming action.

United States Patent Lester et al.

[ 4] STRIPPABLE LAYER RELIEF PRINTING [72] Inventors: Corrsin Lester,138 Highledge Drive, Penfield, NY. 14528; Joan R. Ewing, 107 NormandyAve., Rochester, NY. 14619 52 us. c1. .355/9, 178/66 TP, 340/173 TP,346/74 TP 51 1111. c1. ..G03g 15/00 [58] Field of Search ..355/9; 96/1.1; 346/74 TP; 178/66 TP; 340/173 TP [56] References Cited UNITED STATESPATENTS Dreyfoos, Jr. et al. ..346/74 Wolff ..355/9 [is] 3,692,404 1451Sept. 19,1972

FOREIGN PATENTS OR APPLICATIONS 598,591 4/1961 Belgium ..96/1.1

Primary Examiner-Robert P. Greiner Attorney-Frank A. Steinhilper andStanley Z. Cole 5 7] ABSTRACT Overcoated layer relief electrostaficprinting in which a latent electrostatic image is made visible by thedeformation of a compliant layer. The relief deformation occurs in athermoplastic layer superimposed on conventional xerographic materialssuch as the conductive substrate which has been coated with aphotoconductive insulating layer, the thermoplastic material overcoatingthe photoconductive layer. Apparatus is disclosed for separable andpermanent thermoplastic overcoating on the xerographic plate. Aninterlayer between the thermoplastic overcoating and the photoconductivelayer is included which serves as a deformable support and to protectthe photoconductive layer from any interaction between the particularthermoplastic used and the solvent or heat used to initiate thethermoplastic deforming action.

8 Claims, 11 Drawing Figures PATENTEDSEP 19 I972 SHEET 2 BF 2 VOLTAGESUPPLY STRIPPABLE LAYER RELIEF PRINTING This is a division ofapplication, Ser. No. 193,129 filed May 8, 1962 is now US. Pat. No.3,615,387.

This invention relates to electrostatic printing and, in particular, toforms of electrostatic printing in which the latent electrostatic imageis made visible by the deformation of a compliant layer. In xerography,as it was taught for example by Carlson in U. S. Pat. No. 2,297,691, aninsulating photoconductive layer was sensitized by charging to anelectrostatic potential and then the latent electrostatic image wasformed by exposing the layer to an image pattern of light and shadow toselectively dissipate the electrostatic charge. The latent electrostaticimage thus formed has been conventionally developed by means of anelectroscopic pigmented powder. The powder image then must be fixed to asecond layer or transfer sheet in order to prevent disturbance of thepowder image. These steps of development and fixing of the image aretime consuming and require considerable complexity in the apparatus.More recently, attempts have been made to develop latent electrostaticimages by deformation of compliant layers as produced by theelectrostatic forces of the image. This eliminates the necessity of adeveloper material, reduces the development time, and the complexity ofthe equipment. However, conventional xerographic materials and methodshave not been found to lend themselves readily to this type ofdeformation imaging and attempts to make use of the more obvious methodshave produced weak and impermanent images. Attempts to provide adequatedeformation images have led to systems of increasing complexity. Forexample, systems operating in a vacuum and systems using a deformableliquid with a further development or transfer stop to render itpermanent. In some instances, it is particularly desirable to producehigh resolution images so that large quantities of image data may bestored in a relatively small space or on a relatively small amount ofrecording material. Thus, for example, when recording equipment is usedin different types of missiles and space vehicles, it is desirable thatthe amount of recording material needed to store a given amount ofinformation be relatively small and that the equipment necessary toproduce the image be likewise small without unnecessary operativestages. The necessity in conventional xerography of a bulky developmentstage and of relatively high heat fixing with its attendant high powerconsumption has ruled it out in the past for purposes of this nature.

Now in accordance with the present invention, it has been discoveredthat deformation images can be produced by xerographically deformingsoftenable films temporarily overcoated on conventional xerographicsensitive members. It has further been discovered that the use ofappropriate support layers for said films enables the deformed films tobe readily separated from the sensitive member while preserving theimage. Thus it is an object of the invention to define self-supportingdeformable overlayers for xerographic imaging.

It is an additional object of the invention to define methods fordeformation printing using a photoconductive insulating layer coatedwith a separable deformable member.

It is an additional object of the present invention to define apparatusfor xerographically deforming a separable member.

Further objects and features of the invention will become apparent whiletending the following description in connection with the drawingswherein:

FIG. 1 is a diagrammatic illustration of charging a thermoplastic coatedxerographic plate;

FIG. 2 is a diagrammatic illustration of exposing a sensitizedthermoplastic coated xerographic plate;

FIG. 3 is a diagrammatic illustration of a second method of exposing asensitized thermoplastic coated xerographic plate;

FIG. 4 is a diagrammatic illustration of a second charging step employedin accordance with an embodiment of the present invention;

FIG. 5 is a diagrammatic illustration of simultaneous charging andexposing of a thermoplastic coated xerographic plate;

FIG. 6 is a diagrammatic illustration of vapor development of adeformation image;

FIG. 7 is a diagrammatic illustration of heat development of adeformation image;

FIG. 8 is a further embodiment of heat development of a deformationimage;

FIG. 9 is a diagrammatic illustration of simultaneous exposure anddevelopment of a thermoplastic coated xerographic plate;

FIG. 10 is a diagrammatic illustration of an embodiment using a coloredthermoplastic layer in accordance with the present invention; and,

FIG. 11 is a diagrammatic illustration of apparatus for formingdeformation images on a separable thermoplastic layer.

Some thermoplastic materials have been found to deform readily whensoftened while under the influence of a latent electrostatic image. Anassembly of a xerographic plate carrying a layer of such a thermoplasticmaterial is illustrated in FIG. 1 This arrangement is adapted inaccordance with the invention to sustain either voltage gradients orelectrostatic charge density gradients on a surface which is thendeformable to accordance with such gradients. The plate is shown ancomprising conductive substrate 10 coated with photoconductiveinsulating layer 11 as is conventional. Over the photoconductiveinsulating layer is interlayer 12 which is, in turn, coated withcompliant thermoplastic 13. Substrate 10 may be any conventionalconductive backing as used in conventional xerography. Thus, it may bebrass, aluminum, or other metal or it may be a flexible conductivematerial such as conductive paper or a plastic material coated with aconductive coating such as tin oxide or copper iodide or it may be atransparent material such as glass or clear plastic with a conductivecoating of tin oxide, copper iodide, or the like for transparency. Anyconventional photoconductive insulator such as vitreous selenium,anthracene, sulphur, zinc oxide in a binder material, or otherphotoconductors may be used in insulating binders. However, as will bedisclosed below, photoconductors adapted to forming uniform homogeneouslayers have been found preferable for high resolution purposes.Interlayer 12 serves as a barrier layer between the thermoplastic andthe photoconductive insulating layer and also serves other importantfunctions. It protects the photoconductor from any interaction with theparticular thermoplastic used. It serves as an isolation layer duringdevelopment to protect the photoconductor from the effects of thesolvent vapor or the effects of the heat and at the same time, helps tomaintain electrical insulation between the thermoplastic layer and thephotoconductive layer. A further function of interlayer 12 is inseparable deformation layers in which case the interlayer serves as aseparation support. This is essential since suitable compliant layerssuch as the various insulating thermoplastics have inadequatedimensional stability as self-supporting layers to maintain anundistorted image during separation. Since some photoconductivematerials such as many of the organic photoconductors show nodeleterious reaction to most thermoplastic materials or to temperaturesused'for softening such materials, the use of interlayers with themserves no purpose unless separation is required. Many of the highmelting point plastics are suitable for use as interlayer 12. They arepreferably tough, electrically insulating, and highly transparent Highdimensional stability is required where used for separable layers. Insome embodiments of the invention, as will be seen below, however, theinterlayer need not be transparent. One preferred material is vinylite(trademark of Carbide and Carbon Chemical Company, New York, New York.)polyvinyl chloride. This has been found preferably because of its highinsulating qualities, low reactive effects, high tensile strength, and asoftening point above the temperatures necessary for deforming lowmelting point thermoplastic materials as found suitable for use with thepresent invention. Also suitable for interlayer 12 are other polyvinylchloride or polyvinyl acetate resins, or mixtures thereof, as well aspolyethylene terephthalate and other plastics having the desiredcharacteristics set forth above. Thermoplastic layer 13, in accordancewith the present invention, must be adequately insulating to support anelectrostatic charge on its surface and is preferably selected to becapable of maintaining such a charge while it is softened by heat orvapor to a point where deformation can occur. It is further preferablethat the thermoplastic have a low softening temperature so that it willbe deformed from the effects of a latent electrostatic image attemperatures below about 140 F. It is further desirable that thethermoplastic be free from flow effects at normal room temperatures,that is, below about 90F. A preferred material has been found to beStaybelite (trademark of Hercules Powder Company, Wilmington, DelawareEster No. 10. This material has been found preferable due to longer termstorage characteristics for preserving the image than has been found inother thermoplastics having similar electrical resistance and softeningtemperatures. Other suitable materials are Piccolastic" (trademark ofPennsylvania Industrial Chemical Corporation, Clairton, Pennsylvania),Type A with melting point from 50-75 C.; Nevillac soft (trademark ofNeville Company, Pittsburgh, Pa); and other transparent thermoplasticresins having a melting point generally between 40and 80C. andelectrical resistivity of at least ohm-centimeters at 30C. Thethermoplastic layer and interlayer are preferably kept thin for highresolution and in the case where the layers are permanently bonded, theinterlayer may be as thin as one-tenth of a micron. Where separablelayers are used, the interlayer must be thick enough to provide thenecessary strength and dimensional stability for separation. Thus, forseparable layers interlayer 12 may vary between a few microns and aboutl mil depending on the strength of the material used. The thinner layersmay be applied to the photoconductive insulating layer by permanentlybonding in a dip, spray, or whirl-coating procedure or by vacuumevaporation. For dip, spray or whirl-coating the plastic is dissolved ina solvent and applied to the photoconductive layer in a liquid form andthen hardened by evaporation of the solvent. The thermoplastic layer maybe coated over the interlayer in a similar manner. Where separablelayers are used, the interlayer is preferably in the form of aself-supporting web which is coated with the thermoplastic layer by oneof the procedures suggested above.

The process steps to form the image reproduction in accordance with theinvention are capable of various manipulations which are generallyselected in accordance with the particular conditions and desiredresults. FIG. 1 shows a conventional preliminary charging step that maybe used to sensitize the thermoplastic coated plate of the invention.Corona charging device 15 connected to potential source 16 is arrangedto apply a voltage of between approximately and 1,000 volts to thesurface of thermoplastic layer 13. While either positive or negativecharging may be used, positive charging is illustrated as indicated bythe plus signs shown at the surface of the thermoplastic with matchingnegative charges shown by minus signs in the substrate 10.

FIG. 2 illustrates exposure to an image pattern of light and shadow. Thethermoplastic layer need not be transparent in which case, exposure ismade through substrate 10. Substrate 10 in FIG. 2 is illustrated as atransparent glass or plastic layer with transparent conductive coating17 to enable exposure of the xerographic plate through the back. Thistype of exposure has the advantage in the present invention in that theinterlayer 12 and the thermoplastic layer 13 may have poor opticalqualities and may be colored to the extent of being opaque if desired.It has been found generally preferable to obtain opacity of the plasticcoated side of the plate by coloring interlayer 12. Thus, interlayer 12may be colored by nigrosine dye, for example, which will produceadequate opacity in a 10 micron layer of polyvinyl chloride if added inthe proportion of about 10 to 20 percent weight by volume of nigrosineto plastic. Addition of most colorants in sufficient strength of produceopacity in the deformable layer has generally been found to reduce thebulk resistivity to an excessive degree. If the thermoplastic layer andthe interlayer are opaque, the development step is simplified as will beseen below. In FIG. 2, an image 18 is projected through an opticalsystem 20 onto the xerographic plate. The crosshatched section 21 of theprojected image indicates a dark section with little or no illuminationwhile the uncrosshatched section of the projected image 22 is a light orhigh illumination portion of the image. Where illumination reaches thephotoconductive layer 11, the resistance of the layer decreases so thatnegative charges in the substrate pass up through the photoconductor tothe interface between the photoconductor and interlayer 12. Where thephotoconductor is illuminated, the electrical capacity between thesurfaces bearing the opposite electrical charges in increased due to thedecrease in spacing between the charge carrying surfaces. Increasing thecapacity in this way without changing the charge quantity decreases thevoltage of the charged surface in accordance with the formula Q CE. Qrepresents the quantity of electric charge in coulombs, C equalscapacity in farads, and E represents voltage. It will be seen that whenthe capacity (C) is increased while the charge quantity (Q) ismaintained constant, that the voltage (E will be reduced. Thus, themeasurable potential on the surface of the thermoplastic becomes lessover the illuminated areas than over the dark areas.

FIG. 3 is an alternative embodiment of the exposure step in which theimage pattern of light and shadow is projected onto the photoconductorthrough the thermoplastic layer. As is obvious, this requires a highdegree of transparency in the thermoplastic layer and in any interlayerthat exists. After exposure, the image may be developed immediately orthe voltage differentials existing on the surface of the thermoplasticlayer can first be changed to variations in charge density.

FIG. 4 illustrates a procedure for changing the voltage gradients intovariations in charge density. This is done by repeating the chargingstep as performed in the first sensitization of the plate. Since thecharging devices conventionally used in xerographic processes arevoltage responsive, the charging device sees the reduced voltage overthe illuminated areas and applies more charge as indicated by the doublerow of plus signs over the previously exposed areas of the plate. In theareas where the plate was dark during exposure, the charging device seesthe original voltage and applies no additional charge. Thus, the chargequantity is increased only in the areas that were illuminated during theexposure step. There is a significant difference between the forcespresent after a second charging as in FiG. 4 compared with those presentimmediately after the exposure step. With just the voltage gradients onthe surface, only an edge effect image can be produced while after thesecond charging, it is possible to produce effects on larger areas. Thiswill be described in more detail in connection with image developmentillustrated in FIGS. 7 10.

It is possible to simultaneously charge and expose a thermoplasticcoated xerographic plate as illustrated in FIG. 5. This produces thesame effect as shown in FIG. 4 to a pronounced degree. Thus, since theexposure is going on continuously during charging, charges of onepolarity in the substrate may continuously drift up through thephotoconductive layer in the illuminated areas permitting increasedcharging in the respective thermoplastic surface areas. This permitsgreater rela tive charge density in the illuminated areas as compared toprocesses described in connection with FIG. 4 in which the conductivityof the photoconductor is shut off during the second charging. While inFIG. 5,, the image is illustrated as projected from the same side of thecoated xerographic plate as that on which the charge is applied, it is,of course, possible to project the image through a transparent substratein the manner of FIG. 2 while simultaneously charging the surface of thethermoplastic layer.

Deformation of the thermoplastic layer in the image pattern can beproduced by two general methods. One

is to soften it by heating and the other is to apply a solventpreferably in a vapor form to soften the layer. Heat is consideredpreferable since it is more readily controlled and its action can bestopped more rapidly than that of the solvent. Following exposure as inFIGS. 2 and 3, deformation development must be performed with thephotoconductor shielded from light. If exposure has been made through atransparent substrate and an opaque plastic layer shields thephotoconductor on the side of the deformable layer as has been suggestedabove, thermoplastic layer 13 may be developed by heat or vapor whileunder illumination. Also where recharging has produced charge densityvariations on the deformable surface, development may be carried outunder normal illumination.

FIG. 6 illustrates the use of the solvent vapor. The plate carrying thethermoplastic layer can be passed into chamber 25 containing a solventvapor for the thermoplastic. With a thermoplastic layer of Staybelite,suitable solvents are eythylene dichloride, carbon tetrachloride,hexane, trichloroethylene, or the like.

FIG. 7 and 8 show development by means of heat. The heat source in FIG.7 is indicated as an infra-red lamp 26 and the heat source in FIG. 8 isillustrated as an electrical resistance heating element 27. The infraredheat source is particularly suitable when one of the plastic layers iscolored and exposure is made through a transparent substrate. Thecoloring absorbs the infrared radiation giving preferential heating.Accordingly, interlayer 14 in FIG. 7 is illustrated as an opaque layer.

It is also possible to develop an image by softening the thermoplasticlayer during the exposure step. This is illustrated in FIG. 9 in whichexposure from image 18 is made through transparent substrate 10 while anelectrical resistance heating element 27 applies softening heat to thesurface of the thermoplastic layer.

The amount of heat or solvent to be applied will depend upon thecharacteristics of the thermoplastic layer and of thickness. Staybeliteby way of example, should generally be heated to a surface temperatureof about 45 C. In any case, the viscosity of the material should bereduced to between about 10" to 10 poises. A viscosity below this rangegenerally produces a loss of surface charge which may be due to mobilityof ions in the material as it becomes more fluid. A viscosity above thisrange will still permit deformation, however the time required will runinto several seconds or even minutes which is generally excessive forpractical use. It should also be noted in this connection that repeatedheating of vitreous selenium to temperatures above 50 C. will lower itselectrical resistance. However, with other photoconductors, such as theorganic photoconductors, the repeated use of high temperatures has nosignificant effect on electrical characteristics. In a least oneembodiment of the invention, a lower electrical resistance in seleniumis not necessarily harmful as will be seen below.

In a particularly compact embodiment of the invention, the process stepsof charging, exposure and development are carried out simultaneously asillustrated in FIG. 10. A further discussion of this embodiment is givenin connection with techniques for enhancing image visibility.

the dielectric constant. That is,

ph lh th ph and ph Dh rab where E is the field, Q/A the charge per unitarea K the dielectric constant, layer. the layer thickness, and ph andth the subscripts for the photoconductive and thermoplastic layers,

For typical xerographic use, the potential across a ZO-micron seleniumplate is about 600 volts, so that E 600/(2 X 10') 300,000 volts/cm.

and across the thermoplastic with about one-third the dielectricconstant,

E 900,000 volts/cm.

After exposure, the field in the photoconductor would be reduced to avalue proportional to the induced charge remaining on the substrate, sothat a fully exposed area will have zero field within it. On the otherhand, the field across the thermoplastic does not change (in largeuniform areas). What does change is the potential. The potential of thefree surface is given y suriace 4 th th ml: uh

where =0 the initial charge and 0' remaining on the substrate afterexposure. If now the plastic is softened, nothing will happen in largeexposed areas, because there has been no change in electrostatic stress.However, at the boundary between a region of higher potential(unexposed) and lower potential (exposed) an additional electrostaticfield will be generated on both sides of the edge.

This will create additional electrical and mechanical stress at theexposed edge and reduce stress on the dark side of the edge, to givedeformation in the softened film as shown, for example, in FIG. 7.

As part of an extensive computer analysis of fields above electrostaticsurfaces, a calculation yields a value of 6 X 10 volts/meter for thenormal components of the field at an edge between charged and dischargedportions of the plate. For such a field and a charge density of 1.4 Xcoulombs/cm, the deforming pressure is p= 6 X 10 X 1.4 X 10' 800newtonslm 8,000 dynes/cm For a line electrostatic image 1.0 cm long and0.1 cm wide, this yields a force of 80 dynes.

It should be noted that when a simultaneous development and exposure isused as in FIG. 9, a slightly enhanced image is produced since the firstdisplacement of the surface during development produces additionalvariations in the layer capacity at the image edge increasing thecontrast effected by the exposure and thus permitting a greaterdeformation.

As implied by the above theory of operation, in FIGS. 7, 8 and 9 asillustrated, an edge deformation of the image occurs at the position ofthe potential gradients 28. While this method will not reproduce solidareas, this edge effect type of image is capable of very high resolutionand can be readily projected by the use of Schlieren optics or the like.

Where solid area reproduction is desired, a modification of thereproduction process has been found to permit limited solid areadeformation. An example of this modification is the second charging stepas illustrated in FIG. 4, or in a simultaneous charge and expose methodas in FIG. 5. Thus, if the exposed material is recharged to bring it touniform potential, the field produced by the charge density is increasedin the exposed area. The image response of the softened plastic isgenerally to depress and create large thinner areas whose surfaces areparallel to the original surface. The image on such a layer yields phasedifferences which can be observed by a phase contrast method, howeverthe ability of the material to be squeezed out of an area by theimage-dependent electrostatic force is greatly influenced by theconditions in the surrounding areas in accordingly this method is mostuseful where the areas to be depressed are relatively small. Inreproducing continuous tones or large solid areas, a screening processis preferred to break the large solid image areas into readily deformedsmall areas.

With increased charge density in the exposed areas, a solid areadeformation can be produced as indicated by the depressed areas 30 inFIG. 6. While development of the solid area deformation is illustratedin FIG. 6 by solvent vapor and while the edge deformation developmenthas been illustrated in FIGS. 7, 8 and 9 by heat, it is completely amatter of choice which form of development is used for either the solidarea deformation or the edge deformation. As has been previously stated,heat development is generally preferable in both instances since it ismore readily controlled.

The solid area of deformation produced by differences in charge densityproduces an image of plane parallel areas at different levels. This typeof an image is not readily observable and requires a phase-sensitiveimaging system for display purposes. techniques for enhancing visibilityof the deformed image have been found, however, that permit readyobservation of such an image. FIG. 10 shows an example of this in whichdeformable thermoplastic coating 13 is of contrasting color or of highlydifferentiated color density relative to interlayer 12. Thus, forexample, layer 12 may be transparent while layer 13 is colored as by theaddition of a small amount of nigrosine. These layers can be readilyapplied to the plate by dip coating steps in which layer 12 is permittedto harden and dry before the application of layer 13. Upon forming anddeveloping a solid area image of different charge densities, the exposedareas of the uppermost layer 13 are depressed and thus thinned out tothe point where it is Several virtually invisible and the lower layer 12is exposed to observation. This produces an immediate viewable image. Itis also possible with separable layers to obtain a transparency. Thedeformable thermoplastic layer colored by some colorant such asnigrosine dye is coated on a separable interlayer that is highlytransparent. After image formation and development, the depressed areasof the thermoplastic layer being relatively thin contain relatively lessdye and transmit more light than the areas that are not depressed.Accordingly, the interlayer can be stripped off the plate carrying thedeformed, dyed, thermoplastic layer and utilized in a conventionalprojector. Due to the effect of the usual colorants in loweringresistivity of the thermoplastic it has been found desirable when usingdyed deformable layers to charge, expose and develop simultaneously.Since this requires minimum storage time for the electrostatic chargeson the deformable surface, a substantially lower bulk resistivity iscompatible. With this simultaneous processing, resistivities as low as10 ohm-cm in the deformable layer have still permitted imagedeformation. The illustrated embodiment, FIG. 10, is arranged to provideexposure through substrate 10 while charging and developing from theopposite side of the layered assembly. While this embodiment has beenchosen for ease of illustration, it is just as suitable to use an opaquesubstrate and expose, charge and develop simultaneously from the sidefacing the deformable surface. Substrate l and photoconductive layer 11are the same as described in previously disclosed embodiments.Interlayer 12 is preferably a clear plastic and layer 13 is athermoplastic having a lower softening temperature than layer 13. Forexample, layer 12 can be polyvinyl chloride and layer 13 can bePiccolastic A-75 Layer 13 contains a dye such as nigrosine. Effectivecoloring in a five micron layer of thermoplastic is provided by aboutpercent by weight of nigrosine base per volume of thermoplastic (CGSunits.) Thinner layers require higher percentages of nigrosine andthicker layers require lower percentages of nigrosine to obtain the samemaximum image density.

Heating elements 33 are shown in association with charging device 15. Asthe charging device is operated to apply an electrostatic charge, theheating elements function to heat the same area to the deformationtemperature of deformable layer 13. Source of illumination 34 isoperative in conjunction with optical system 20 to project a light andshadow pattern of image subject 18 onto photoconductive layer 11.Voltage source 29 applies operating potential to charging device 15,heating elements 33, and source of illumination 34 simultaneously by aganged switch 39. This simultaneous method has been found to be fast andis adapted to compact systems.

A method that avoids the use of colored layers requires an extradevelopment step. By this method, a depressed area image is formed byany of the processes previously discussed and then a high viscosity orpastelike pigmented material is wiped over the surface of the deformedplastic so that it fills in the depression. Pigmented materials thathave been found useful for this purpose include printers ink and many ofthe graphite dispersions sold under the trademark Dag such as Aquadag byAcheson Colloids Corporation of Port Huron, Michigan.

A reusable temporary overcoating system is illustrated in FIG. 11. Thisfigure shows the continuously operable apparatus for producing deformedthermoplastic images on a thermoplastic layer overlying a continuousphotoconductor web. The photoconductive web 35 comprises aphotoconductive insulating layer on a conductive backing material whichis carried onto rotatable cylinder 36. Cylinders 36 are connected forrotation to a drive means 49. Arranged in sequence in the direction ofrotation of the photoconductive web is erasing station 37, chargingstation 38, exposure station 40, recharging station 41, developmentstation 42 and separating station 43. The thermoplastic layer 45 coatedon a heat resistance transparent plastic support member 46 is fedthrough the erasing station 37 and into traveling contact with thephotoconductive web by feed means 44. The surface of photoconductive web35 is precharged at electrostatic charging station 38 before contactingplastic support member 46. At erasing station 37, heat or solvent vaporis applied to smooth out the surface of the thermoplastic and erase anyimages on it that may remain from previous use. This erasing station mayalso suitably include cooling or drying means so that the thermoplasticlayer will be more highly insulating when advanced over photoconductiveweb 35. The plastic support 46 carrying thermoplastic coating 45 istransported along with the movement of the photoconductive insulatinglayer under pressure roller 53. Pressure roller 53 is a conductiveroller with or without an insulating surface layer and having anelectrical connection to reference potential. The electrical referencepermits the roller to apply electrostatic pressure as well as mechanicalpressure to assure a uniform contact between member 46 and web 35. Thelayers are then transported together past the exposure station 40 whichsuitably employs a conventional moving slit exposure means operating insynchronization with the movement of the layers. The exposure stationprojects a pattern of light and shadow through the thermoplastic and itssupport onto the photoconductive insulating layer 35 in accordance withan image subject 47. The latent electrostatic image thus formed appearsas voltage gradients on the surface of the thermoplastic insulatinglayer. The combined layers then pass through the second charging station41 where residual conductivity in the previously illuminated areas ofthe photoconductive layer permits enhanced variations in the chargedensity produced by the voltage sensitive charging device. After thesecond charging, a development station 42 using heat or a solvent vapordevelops the charge density variations on the thermoplastic layer. As inthe case of erasure station 37, development station 42 suitably includescooling or drying means to harden or fix the thermoplastic layer so thatthe deformation image will remain after removal of the electrostaticimage-forming field. The thermoplastic layer along with its supportlayer have been separated from the photoconductive insulating layer andutilized as by a Schlieren optical system for projective of the image.When the deformable layer is not permanently bonded to the xerographicmember, as in FIG. 11, it is preferred to wet the surface of thexerographic member before applying the plastic layer. Such wetting helpsto eliminate air bubbles and may be added in a washing process thatreduces dust or lint buildup on the xerographic plate. Silicone oil suchas type DC-200 -20CS (Dow Corning), other light oil or any electricallyinsulating low viscosity liquid that does not chemically react with thexerographic plate or the plastic layer can be used. FIG. 11 shows bath50 for applying a liquid film to xerographic web plate 35.

The present invention has a particular advantage in high resolutionreproduction for high density image storage and the like. Resolutionsgreater than 115 line pairs per millimeter have been obtained. Foroptimum resolution, certain materials and processes are preferred. Thephotoconductive material, itself, is preferably selected to have asmooth homogenous surface when coated on a substrate. Suitablephotoconductive coatings are vacuum evaporated selenium or organicphotoconductors dissolved in a solvent with an organic resin material.The organic solution provides a smooth homogenous coating by spray,whirl or dip coating procedures. Organic photoconductors for thispurpose include2.5 bis (4 diethyl aminophenyl) l, 3, 4, oxadiazole;2.5-bis-(p-aminophenyl)-l, 3, 4, triazoles and other and other 1, 3, 4,oxadiazole and l, 3, 4, -triazole compounds. One commercially availableexample is Kalle To 1920, available from Kalle and Co.,Wiesbaden-Biebrich, Germany.

The thickness of the layers is a significant factor in high resolutionembodiments. The thickness of the photoconductive layer is not ascritical as the thickness of the overcoatings, but with vitreousselenium the best resolution have been obtained wit a vitreous seleniumlayer of about 50 microns. Layers from about 20 to 80 microns ofvitreous selenium also produced good results. With other homogenousphotoconductive layers such as organic photoconductive layers, highresolutions have been obtained with layers as thin as about theremicrons.

Of greater significance for high resolution considerations is thethickness of material between the photoconductive surface and thedeformable surface. Empirically it has been found that the maximumresolution that can be obtained is generally limited by the thickness ofsuch material in accordance with the relationship r where R representsthe resolution in line pairs per millimeter, K is the dielectricconstant of the material for resolutions of better than 100 line pairsper mm., the thickness of material between the photoconductive surfaceand the deformable surface must be loss than microns thick assuming adielectric constant of about 4. With the thickness of an interlayeradded to the thickness of the deformable thermoplastic between thephotoconductive surface and the deformable surface, the dielectricconstant must be adjusted accordingly.

While the present invention has been described as carried out inspecific embodiment thereof, there is no desire to be limited thereby,but it is intended to cover the invention broadly within the spirit andscope of the appended claims.

What is claimed is:

1. Electrostatic deformation printing apparatus for continuouslydeforming a thermoplastic surface in accordance with illumination of animage pattern comprising:

a. means to support a continuous xerographic plate,

b. drive means for continuously advancing said xerogra phic platemounted on said means to sup or t an ,arranged ll'l operational sequencearoun said means to support,

c. a bathing station for the application of a liquid film to the surfaceof said xerographic plate,

d. feed means adapted to feed a plastic web over said xerographic plate,

e. an erasing station adapted to remove deformities from the surface ofsaid plastic web,

f. a first sensitizing station adapted to apply an electrostatic chargeto at least said continuous xerographic plate,

g. an exposure station adapted to expose said xerographic plate toillumination in an image pattern,

h. a second sensitizing station adapted to apply a second electrostaticcharge to said plastic web,

i. a deforming station adapted to soften said plastic web so that itwill deform in response to a latent electrostatic image, and,

j. separation means for separating said plastic web from saidxerographic plate.

2. Apparatus according to claim 1 wherein said liquid film comprises anelectrically insulating low viscosity oil.

3. Apparatus according to claim 1 wherein said deforming stationcomprises an infrared heat source.

4. Apparatus according to claim 1 wherein said xerographic platecomprises a layer of vitreous selenium of about 20 to microns inthickness.

5. Apparatus according to claim 1 wherein said plastic web comprises adouble layered electrically insulating overlay wherein the layernon-adjacent the xerographic plate is a thermoplastic material having alower softening temperature than the layer adjacent the xerographicplate.

6. Apparatus according to claim 5 wherein one of said layers is ofhighly differentiated color density relative to the other of saidlayers.

7. Apparatus according to claim 5 wherein said thermoplastic layer has asoftening temperature between about 40 and 80 C. viscosity of about 10to 10 poises.

8. Apparatus according to claim 5 wherein the layer adjacent saidcontinuous xerographic plate has a thickness of less than 10 microns.

2. Apparatus according to claim 1 wherein said liquid film comprises anelectrically insulating low viscOsity oil.
 3. Apparatus according toclaim 1 wherein said deforming station comprises an infrared heatsource.
 4. Apparatus according to claim 1 wherein said xerographic platecomprises a layer of vitreous selenium of about 20 to 80 microns inthickness.
 5. Apparatus according to claim 1 wherein said plastic webcomprises a double layered electrically insulating overlay wherein thelayer non-adjacent the xerographic plate is a thermoplastic materialhaving a lower softening temperature than the layer adjacent thexerographic plate.
 6. Apparatus according to claim 5 wherein one of saidlayers is of highly differentiated color density relative to the otherof said layers.
 7. Apparatus according to claim 5 wherein saidthermoplastic layer has a softening temperature between about 40* and80* C. viscosity of about 104 to 106 poises.
 8. Apparatus according toclaim 5 wherein the layer adjacent said continuous xerographic plate hasa thickness of less than 10 microns.